This invention relates generally to ocean thermal energy conversion systems, and more particularly to improved means for dynamically positioning a vessel containing the energy conversion apparatus.
Various proposals have been made for utilizing the temperature difference between the warm surface waters of the tropics and the colder deep-water regions to operate a binary closed Rankine cycle energy conversion system. Such a system is generally known as an OTEC system, which is an acronym for ocean thermal energy conversion. The energy conversion system thermodynamically converts the thermal gradient of the ocean into electrical energy.
The vertical stability of the vessel containing the OTEC apparatus presents no unusual difficulty, because this is easily achieved by conventional hull structure and ballast designs well known in the ship-building industry. However, the horizontal positioning of the vessel that is required to overcome the forces of wind and ocean current poses unique problems because of the large hull displacement and the requirement for a huge cold water pipe that extends to unusual depths.
Several approaches have been considered for achieving horizontal positioning of the vessel by fixed moorings, such as by wire, chain, or synthetic rope. These approaches have several disadvantages in that they require excessive numbers and length of mooring legs, which involves very heavy and expensive equipment.
Dynamic positioning which involves conventional propeller thrusters has also been considered. However, it has been estimated that approximately 20% of the power produced by OTEC would be required to counteract the most extreme wind and current forces that might be encountered. For this reason and for reasons of high initial cost and large vessel space requirements, this method of dynamic positioning has been found lacking. It would be highly advantageous if a method of positioning could be devised which imposes minimum demands on the power plant and vessel configuration.
There are principally two different options which appear attractive for utilizing the electrical power produced by an OTEC system. One usage is the transmission of electrical power to shore, and the other is the in situ use of the electrical power for an energy intensive production process, such as ammonia manufacture, ore refining, and production of fuels such as hydrogen and methanol. The dynamic positioning requirements for each of these options is different in that fixed station keeping is required in the former and propulsion and steering is required in the latter.